The present invention relates to a method for insulating thermal devices, such as electric freezers, refrigerators, refrigerated containers or the like with a low conductivity, evacuated powder insulation.
Numerous materials heretofore have been used to retard heat flow. These materials include such thermal insulations as glass fiber, and organic foams like polystyrene, polyurethane, and polyisocyanurate. A relatively new class of thermal insulators are evacuated, powder-filled panels. A comparison of the thermal insulating properties of these insulating materials is presented in Table 1.
TABLE 1 __________________________________________________________________________ THERMAL THERMAL CONDUCTIVITY RESISTANCE K FACTOR R FACTOR INSULATION INSULATING (BTU-IN/HR- (HR-SQFT-DEG F/ MATERIAL GAS SQFT-DEG F) BTU-IN) __________________________________________________________________________ GLASS FIBER AIR 0.32 3.13 POLYSTYRENE FOAM AIR 0.23 4.35 POLYURETHANE FOAM CFC 0.12 8.33 POLYISOCYANURATE FOAM CFC 0.12 8.33 EVACUATED POWDER VACUUM 0.04 25 INSULATION __________________________________________________________________________
As can be seen from Table 1, those materials containing chlorofluorocarbon (CFC) gas have conductivities that are approximately a third to a half the conductivity of the air filled materials. Partly because of this lower conductivity, CFC containing insulation has replaced air filled insulation in many applications. Evacuated powder insulation has a conductivity that is considerably lower than insulation containing air or CFC gas. Applications of this insulation concept have been slow in developing. This has been caused, mainly, by the high cost to manufacture the evacuated insulation.
Thermal insulation is a key parameter in the worldwide energy equation. A reduction in energy usage was realized when CFC blown rigid foams were developed and used in thermal insulation applications. The discovery that CFC's are reducing the stratospheric ozone and thus increasing the amount of destructive UV radiation reaching the earth's surface, has resulted in an international agreement to restrict the future production of CFC's. If viable alternatives are not developed, these restrictions will result in increased energy demands.
At present the only alternatives to foams containing CFC's are foams blown with carbon dioxide or hydrogen containing chlorofluorocarbons (HCFC's). Carbon dioxide and HCFC blown foams are about 10-33% higher in thermal conductivity than CFC blown foams. For this reason, if either carbon dioxide or HCFC were to be substituted for CFC in urethane foam applications, there would be a significant increase in the worldwide energy consumption. Evacuated powder insulation is about 66% lower in conductivity than CFC blown foams. By making a composite insulation system based on an HCFC or a carbon dioxide blown foam with evacuated powder insulation, CFC's can potentially be eliminated in insulating foam and replaced by systems with lower overall conductivity.
The rate at which heat is transferred through thermal insulation is dependent upon the type and thickness of the insulation. Table 2 shows a comparison of thermal properties for urethane foam blown with CFC and carbon dioxide. The table also shows the thermal properties for each of these foams in a composite insulation system with evacuated powder panels. So as to make a valid comparison, the overall insulation thickness was the same for each of the insulation materials. It is readily apparent from Table 2 that thermal conductivity of any of the composites is lower than the conductivity of either the CFC or the carbon dioxide blown foams.
Since the evacuated powder panels have the largest effect on the conductivity of the composite, it is important that they cover as much of the insulated surface area as possible. Many thermal devices, for example, refrigerators, freezers and walk-in coolers have large flat areas that require insulation. The current evacuation chamber technology is not capable of making panels large enough to cover these large areas with a single panel. As a consequence, these large surface areas require several smaller evacuated powder panels. This results in greater heat leakage through a given wall than would be experienced if the wall were covered by a single panel.
TABLE 2 __________________________________________________________________________ THERMAL THERMAL CONDUCTIVITY RESISTANCE INSULATION K FACTOR R FACTOR INSULATION THICKNESS (BTU-IN/ (HR-SQFT-F/ MATERIAL/GAS (INCHES) HR-SQFT-F) BTU-IN) __________________________________________________________________________ URETHANE FOAM/CFC 1.5 0.12 12.5 URETHANE FOAM/CARBON DIOXIDE 1.5 0.16 9.375 URETHANE FOAM/CFC 1.0 0.072 20.83 EVACUATED POWDER 0.5 URETHANE FOAM/CFC 1.3 0.092 16.30 EVACUATED POWDER 0.2 URETHANE FOAM/CARBON DIOXIDE 1.0 0.082 18.29 EVACUATED POWDER 0.5 URETHANE FOAM/CARBON DIOXIDE 1.3 0.114 13.16 EVACUATED POWDER 0.2 __________________________________________________________________________
This new class of insulating materials, namely, evacuated insulation panels, are costly to manufacture and install in thermal devices. These evacuated panels consist of a finely divided powder, a microporous pouch, and a gas barrier, water-tight envelope.
A method of fabricating evacuated insulation panels is as follows:
1. A microporous pouch is filled with a given quantity of finely divided powder, so as to yield, after compaction, a panel with a density of between 10 and 20 pounds per cubic foot. PA0 2. The pouch is sealed. PA0 3. The microporous pouch containing the powder is placed in an oven and dried for several hours to remove the adsorbed moisture. An alternative to this step, is to dry the powder prior to filling the pouch. PA0 4. After removal from the oven, the pouch is placed in a gas barrier, water-tight envelope and compacted to the desired thickness. This compaction yields a rigid, board-like panel. PA0 5. The rigid, board-like panel is placed in an evacuation chamber, evacuated to a pressure of approximately 2 millibars and sealed.
The finished panels are then, generally, attached, as by adhesive, to the wall(s) of the thermal device to be insulated and then foamed-in-place with a liquid, organic material, such as polyurethane.
It is an object of the present invention to provide a simple method of insulating thermal devices with evacuated powder and organic foams.
It is another object of the present invention to reduce the costs associated with the manufacture and the application of evacuated powder insulation to thermal devices.
It is a further object of the present invention to reduce the heat leakage into thermal devices, such as refrigerators and freezers, by insulating more of the wall area with the evacuated powder than can be insulated with evacuated panel insulation. A secondary effect of greater wall coverage is that there will be less surface heat leakage from the hot side of the evacuated panel to the cold side. This type of heat transfer is often referred to as "edge loss".
Further objects and advantages of this invention will become apparent as the description of the invention proceeds and the novel features of the invention are pointed out in the claims, which form part of this specification.
U.S. Pat. No. 4,636,415, entitled "Precipitated Silica Insulation", Barito et al, Jan. 13, 1987 and U.S. Pat. No. 4,681,788 entitled "Insulation Formed of Precipitated Silica and Fly Ash", Barito et al, July 21, 1987, describe evacuated panels containing precipitated silica and precipitated silica and fly ash. These panels may be used to insulate a space by having the panel form part of the walls surrounding the space to be thermally insulated. These patents teach that these panels are particularly suited for applications where they can be sandwiched between a double wall. Further, they teach that these panels can be used in refrigerators and freezers by bonding the panels to the inside wall of the outer case or the inside wall of the inner liner and then filling the remaining insulation space between the two walls with polyurethane foam.
The high costs of materials, labor, and equipment to fabricate evacuated insulation panels and to assemble them in thermal devices have contributed to the slow growth of this technology. The present invention greatly reduces these costs.
Current evacuated panel fabrication technology is limited to panel sizes that are smaller than the insulation area of most thermal devices. Therefore, it is necessary, when insulating a large area, to use several panels. When the panels are fabricated there are sealing flaps on the four edges. For devices that are to be foamed after the installation of the panels, these flaps can cause voids in the foam due to entrapped air. To minimize this problem, there are generous spaces between panels in the insulation cavity. Because of the spacing of the panels, on most refrigerators, and like devices, the maximum area that can be insulated with evacuated panels is about 50% of the total available insulation area. This results in a greatly reduced area that is insulated with the lower conductivity, evacuated powder insulation. The present invention makes it possible to insulate more than 90% of the available insulation area with low conductivity evacuated powder insulation.